Release and Transport of Bacteria and Nutrients from Livestock Manure Applied to Pastureland

by Soupir, Michelle Lynn

Abstract (Summary)

Transport of fecal bacteria and nutrients from point and nonpoint sources to surface water bodies is of significant concern in Virginia and the United States. In Virginia, 4,320 river miles are impaired for one or more beneficial use and 72% of the streams are impaired due to pathogen indicators (VDEQ, 2002). Land applications of manure from confined animal systems and by direct deposit by grazing animals are both major sources of fecal bacteria and nutrients in runoff. Therefore, an understanding of the overland transport mechanisms for fecal bacteria and nutrients is very important for the development of best management practices to reduce loading of pathogens and nutrients to surface water bodies. The objectives of this study were to quantify the release and transport potential of three fecal bacterial indicators: E. coli, Enterococcus, and fecal coliforms; and nitrogen and phosphorus from land applied manure during runoff events. Another objective was to identify the Enterococcus species present in dairy manure and determine which species have the highest potential to be transported by runoff.
Release plots were established to study the in-field bacteria and nutrient release. The bacteria and nutrients released from the plots are available to be transported to the edge of the field in runoff. Four manure treatments (turkey litter, liquid dairy manure, cowpies, and none or control) and three land type treatments: pasture with a history of poultry litter application (Turkey Farm), pasture with a history of liquid dairy manure application (Dairy Farm), and pasture with no prior manure application (Tech Research Farm) were studied. During a short but intense rainfall event, the highest bacterial release was measured under the cowpie treatment (E. coli concentrations ranging from 37,000 to >300,000 and FC concentrations ranging from 65,000 to >300,000). Pasturelands with a history of previous manure applications did not release higher bacteria concentrations compared with pasturelands which had never received manure applications. Pasturelands with a history of land application of liquid dairy manure and turkey litter had 143% and 94% higher TSS concentrations available to be transported off the field during overland flow events because of the build up of organic material on the soil surface. TP concentrations released from the cowpie, liquid dairy, and turkey litter treatments were 3.12 mg/L, 3.00 mg/L, and 1.76 mg/L, respectively.
Transport plots were developed to measure the concentrations of fecal bacteria and nutrients present in overland flow at the edge of the field. The bacteria flow-weighted concentrations were highest in runoff samples from the plots treated with cowpies (200,000 CFU/100 mL of E. coli and 234,000 CFU/100 mL of FC). The turkey litter had the highest concentration of dissolved phosphorus in runoff from pasturelands (1.22 mg/L), but the cowpie treatment had the highest concentrations of sediment bound phosphorus in runoff (0.73 mg/L). All three treatments investigated in this study contributed to phosphorus loading in surface waters and could potentially increase the risk of eutrophication. Total nitrogen concentrations from the transport plots exceeded the threshold for likely eutrophication problems for all treatments and the total nitrogen concentrations from plots treated with cowpies exceeded the threshold for severe eutrophication problems.
The Biolog System, a method of bacterial source tracking, was used to identify the different species of Enterococcus present both in the cowpie source manure and in the runoff collected from the transport plots treated with cowpies. The source manure is dominated by the Enterococcus mundtii (55%), Enterococcus gallinarum (20%), Enterococcus faecium (10%), and Enterococcus faecalis (10%). Enterococcus faecalis had the highest percentage of isolates present in runoff with a total of 37%, followed by Enterococcus mundtii which was present in 21% of the runoff events and Enterococcus gallinarum and Enterococcus faecium (11%).
Improvements in understanding the bacterial release and overland processes will enhance modeling of bacteria and nutrient transport, and provide a basis for a more realistic evaluation of the impacts of management practices implementation. The data from this study will serve as a baseline to model the release and transport of fecal bacteria and nutrients from agricultural watersheds to surface waters.